Quadrupole mass spectrometers have proven to be general purpose mass analyzers. These devices are four rod structures and, when operated in a resolving mode, the rods are usually about 20 cm in length and require extreme mechanical precision in terms of fabrication and alignment. When operated in resolving mode quadrupole mass spectrometers have both RF and DC voltages applied to them, and are pumped to a relatively high vacuum (e.g. 10.sup.-5 Torr). Values of these voltages vary with the frequency and mass range of operation, but can be on the order of 1600 volts (peak-to-peak) RF for operation at 1 MHz and .+-.272 volts DC for a rod array inscribed radius r.sub.0 of 0.415 cm and a mass range of 600 Daltons. The high degrees of mechanical and electrical sophistication required means that the costs of these mass spectrometers are high.
There has accordingly been a long standing need for a simpler, less expensive mass spectrometer. While costs have been reduced, quadrupole and other rod mass spectrometers (e.g. octopoles and hexapoles) have continued to remain extremely expensive and to require very close tolerances and high vacuum pumping equipment, as well as costly power supplies.
Attempts have been made to simplify the design and operation of quadrupole mass spectrometers, and one proposal is found in U.S. Pat. No. 4,090,075. This patent teaches that a quadrupole mass spectrometer can provide mass resolution in the absence of applied resolving DC voltages. This so called RF-only mode of operation has several advantages over conventional RF/DC operational modes. Conventional RF/DC quadrupole rod mass spectrometers supply mass resolution based on the intrinsic stability or instability of given ions within the rod structure in the combination of the time varying RF and the time independent DC fields. In contrast to the more common RF/DC quadrupole mass analyzers, mass resolution for an RF-only instrument is thought to occur when ions that are only marginally stable with a particular applied RF voltage gain excess axial kinetic energy in the exit fringing field of the rod structure. A large part of the phenomena leading to mass resolution of an RF-only mass analyzer occurs at the exit of the rod array, so the length limitations characteristic of RF/DC resolving quadrupoles no longer apply and mechanical tolerances for rod roundness and straightness are considerably relaxed. Finally, there is no need for a high precision high voltage DC power supply in the RF-only mode of operation. Taken together the inherent advantages of RF-only operation suggest the opportunity for a much smaller and less costly mass analyzer than conventional RF/DC quadrupoles. Although the potential of such a device is significant, problems arise, such as sample dependent background from high velocity ions and clusters. The current invention describes a method for elimination of these background species.
Another proposal is found in U.S. Pat. No. 4,189,640, which describes a method for background reduction for RF-only quadrupole mass analyzers. This invention teaches that a centrally located attractively biased disk of the appropriate size located after the analyzing quadrupole reduces high velocity and higher mass species. However, in practice this also reduces analyte ion intensity offsetting much of the expected gains in the signal-to-noise ratio.